Glutamatergic transmission plays a critical role in physiological and pathophysiological behavior, from learning and memory to drug addiction. A fundamental property of many glutamatergic synapses is that they undergo long-term potentiation (LTP). As a lasting enhancement of transmission in response to transient stimuli, LTP is an attractive candidate cellular mechanism underlying some aspects of learning, memory and drug addiction. Neuromodulatory neurotransmitters play key roles in regulating both these behaviors and glutamatergic synaptic transmission. Indeed, "metaplastic" processes initiated by prior synaptic activity or neuromodulators regulate LTP. However, the mechanisms underlying this "metaplasticity" are unknown. Neuromodulators activate three classes of receptors, those coupled to Gs, Gq and Gi families of heterotrimeric G-proteins. A large body of evidence suggests that Gs-coupled receptors influence a variety of aspects of synaptic transmission, cellular excitability and LTP. This regulation occurs in large part through the recruitment of the cAMP and p42-44 MAP kinase (ERK) signaling cascades in hippocampal neurons. In contrast, relatively little is known of the role of Gi-linked receptors in LTP. However, Gi-linked receptors have been demonstrated to play key roles in learning and memory, as well as drug addiction. Further, like Gs-coupled receptors, Gi-linked receptors can regulate the cAMP and ERK signaling cascades, though in more complex ways. In non-native systems Gi-linked receptors activate ERK and differentially regulate cAMP levels. While it is known that in CAl Gi-linked receptors regulate synaptic transmission and cellular excitability, it is unclear whether native Gi-linked receptors exhibit the complex regulation of cAMP levels and ERK activation seen in the non-native systems. Finally, Gs-coupled receptors are unlikely to be activated independently in vivo, but instead will likely be co-activated with Gi-linked receptors by endogenous ligands. In this application, the roles of Gi-linked receptors in LTP will be addressed using a combination of biochemical, genetic and electrophysiological approaches in mouse hippocampal slices. The coupling of Gi-linked receptors to the cAMP and ERK signaling cascades will be examined in area CAl of mouse hippocampus using a combination of biochemical, electrophysiological and imaging approaches. In parallel, Gi-linked receptor regulation of LTP, particularly those forms that require the activation of these cascades, will also be examined.